**Core Concept**
The most important antigen initiating graft rejection is a major histocompatibility complex (MHC) molecule, specifically human leukocyte antigen (HLA) class I or II. MHC molecules are highly polymorphic and play a critical role in the immune system's ability to distinguish between self and non-self.

**Why the Correct Answer is Right**
The immune system recognizes graft-derived MHC molecules as foreign and mounts an immune response against them, leading to graft rejection. HLA class I molecules are expressed on the surface of all nucleated cells and present endogenously synthesized peptides to CD8+ T cells. HLA class II molecules are expressed on the surface of antigen-presenting cells (APCs) and present exogenously derived peptides to CD4+ T cells. The recognition of graft-derived MHC molecules by T cells triggers a cascade of immune responses, including T cell activation, proliferation, and cytokine release, ultimately leading to graft rejection.

**Why Each Wrong Option is Incorrect**
* **Option A:** While other antigens, such as ABO blood group antigens, can also initiate graft rejection, they are not as critical as MHC molecules.
* **Option B:** Cytokines play a role in graft rejection, but they are not the primary antigen initiating the immune response.
* **Option D:** Complement components are involved in the immune response, but they are not the primary antigen initiating graft rejection.

**Clinical Pearl / High-Yield Fact**
MHC matching between donor and recipient is crucial in reducing the risk of graft rejection, particularly in organ transplantation. ABO blood group matching is also important to prevent hemolytic reactions.

**Correct Answer:** C. MHC molecules are the most important antigens initiating graft rejection.